A radar is an electronic system that is designed to detect a target and measure the range, angle, and velocity of the target. It performs these functions by transmitting RF signal pulses through an antenna that focuses the energy into a narrow beam to illuminate the target. Range is calculated by measuring the elapsed time between the transmitted pulse and the target echo detected by the radar receiver.
In a conventional Pulsed Doppler (PD) type radar, for example, the target radial velocity component is calculated from measurements of the difference frequency between the transmitted signal and its echo.
Aircraft may be equipped with devices used to transmit RF ECM signals for the purpose of corrupting the target information produced by the defensive radar. Against vintage radar design, such ECM devices have generally been effective because the radar transmits short time duration (typically <1 μsec.) signals. For each radar pulse, the ECM device can transmit almost an unlimited number of signal pulses. In the presence of a large number of ECM pulses a pulse tracker performing range tracking functions becomes overloaded with many ECM pulses that look like target echoes, thus rendering the radar system ineffective.
ECM techniques to counter pulse compression (PC) radar systems are more complex because a PC radar transmits long duration pulses. In the PC radar receiver, the target echo is compressed to a narrow spike. The range tracker receives a narrow lobe comparable to a high peak power pulse. The ECM device cannot physically produce “n” signal pulses for each pulse received from the radar, especially when several PC radars are to be countered.
Spread Spectrum (SPSP) radars are practically immune to conventional range and velocity deception techniques that are based on repeating radar signals. Within the class of PC radars there is a sub-set of radar systems that employ a linear frequency modulated (LFM) signal, often called ‘Chirp’.
Driven primarily by limitations on the manufacturing ability to produce greater and greater peak power transmitters for radars to operate over long ranges, pulse compression signal processing techniques were incorporated in a few target tracking radars in the early 1960's. With the advances made in hardware technology, long range low peak power spread spectrum radars began to appear thereafter and the number of such radar systems is steadily increasing. The spread spectrum signals used in long range surveillance radars are predominantly LFM PC signals. The PC signal waveform is characterized by a time bandwidth product (TB>>than 1), or pulse compression, that is typically several hundred.
There are good reasons for using LFM signals with low compression ratios. To maintain a high probability of target detection and low false alarm rate, a large signal to noise power ratio must be maintained. The receiver noise bandwidth of the most common long range radar is on the order of 10 MHz. There are limitations on the signal time duration. The radar receiver must be blanked for the emission duration of the transmitter high power signal. If the transmitted signal envelope time duration is T, the Matched Filter (MF) response extends over 2T. For a target at a range time T/2, the radar receiver produces output (MF) at the end of 3T past the main long pulse. The radar transmitter must be put in a standby mode for 2T past the trailing edge of the emitted PC waveform. Also, the magnitude of the LFM signal compression attribute is not arbitrarily chosen. Rather, it must satisfy the requirements of the signal to noise power radio (S/N), which decreases with increased receiver (compression) filter bandwidth.
The well-known range, velocity, and angle deception ECM techniques that were developed to counter the vintage radar designs are considered to be just as effective when applied to PC radars. The signals that are employed in these techniques, however, are vastly different. In deception of PC radars, the characteristics of the radar signal waveform have to be altered, in addition to the amplitude modulation, time delay, and pulse to pulse RF phase shifts that are fundamental in all electronic deception techniques.
LFM Signal Properties
A PC radar functions just like a traditionally designed radar of the same class. The only difference is in the signal waveform designed for transmission and the processing performed in the radar receiver on the signal returned by the target radar cross section. The signal emitted by a PC radar has low peak power, long time duration, and linear frequency modulated (LFM) over the radar receiver operational frequency band. The radar antenna directs the transmitted signal toward an airborne target (airborne moving platform), and a small fraction of the energy is reflected by the target back to the radar antenna and its output signal is guided to the radar's receiver for processing. The transmitted signal is frequency modulated by a linear function that is typically many microseconds long. In the receiver, the target return signal is compressed by a matched filter process. The filter output is defined by the signal auto-correlation characterized by one narrow central lobe and many low level secondary lobes. The central lobe is 1/Δf wide at the 50% (3 dB) power points, where Δf is the frequency modulation bandwidth. The power of the returned signal is distributed over a time span equal to two times the transmitted signal duration (2T), with 50% of its power confined to the spike region 1/Δf.
An elementary block diagram of a basic PC radar system 100 is presented in FIG. 1. Clearly indicated is the long time duration frequency modulated pulse 102 and its compressed version 104 appearing as the matched filter 106 response that constitutes the receiver output 108. The filter response is represented by a narrow spike 110 located at the center of the output. Whereas the LFM signal time duration is T (10 in this example), the matched filter output is distributed over 2T time duration (20 in this example). It should be clearly noted that, while the filter response is being formed, the transmitter 112 is kept from radiating high receiver signals.